In recent years, with the widespread use of a mobile device such as a mobile phone and a notebook computer, the role of a secondary battery serving as a source of power thereof has been of importance. Such a secondary battery is demanded to be small and light, and to have the performances of having a high capacity and being hardly degraded even in repeating charge-discharge. Currently, a lithium ion secondary battery is most frequently used as the secondary battery.
Carbon such as graphite and hard carbon is mainly used for the negative electrode of the lithium ion secondary battery. While carbon can allow a charge-discharge cycle to be favorably repeated, it is used in a capacity that has already reached around the theoretical capacity, and therefore a significant increase in capacity cannot be expected hereafter. On the other hand, the lithium ion secondary battery is strongly demanded to have an enhanced capacity, and a negative electrode material is studied which has a higher capacity, namely, a higher energy density than carbon.
Now, silicon as a negative electrode material that can achieve a high energy density is exemplified. Non Patent Literature 1 describes the use of silicon as the negative electrode active material. The negative electrode in which silicon is used as the negative electrode active material is large in the amount of a lithium ion to be stored and released per unit volume and has a high capacity, but the negative electrode active material by itself is considerably expanded and contracted in storage/release of the lithium ion. Therefore, pulverization of the negative electrode active material proceeds. Thus, there is a problem of separation between the negative electrode active material and a foil as a negative electrode current collector. Accordingly, there is also caused problems of an increase in electrode resistance and of a decrease in charge-discharge cycle life.
In order to solve such problems, Patent Literatures 1 and 2 disclose a technique of diffusing an active material in a current collector to form a solid solution.
In addition, Patent Literature 3 discloses a negative electrode in which a negative electrode active material layer, which forms an alloy at an interface with a negative electrode current collector having a protrusion and which includes at least one of elemental silicon and a silicon compound, is provided on the negative electrode current collector.